Higher-order chromatin structures are of profound importance in gene regulation and epigenetic inheritance (Wu and Grunstein (2000) Trends Biochem. Sci. 25:619-623). Post-translational modifications of core histones influence the establishment and maintenance of higher-order chromatin structures. The unstructured tails of certain core histones are extensively modified by acetylation, methylation, phosphorylation, ribosylation and ubiquitination. A “histone code” hypothesis, linking histone modifications to chromatin structures, has been the focus of intensive recent studies (Strahl and Allis (2000) Mol. Cell. Biol. 22:1298-1306; Turner (2000) Bioessays 22:836-845). Histone methylation has emerged as a major form of histone modification. (Strahl and Allis (2000) Mol. Cell. Biol. 22:1298-1306; Zhang and Reinberg (2001) Genes Dev. 15:2343-2360). In particular, a large family of SET domain-containing histone methyltransferases (HMTases) has been identified (Lachner and Jenuwein (2002) Curr. Opin. Cell Biol. 14:286-298). SET domain proteins have been shown to methylate various N-terminal lysine residues of histone H3 and H4. Histone lysine methylation has been associated with diverse biological processes ranging from transcriptional regulation to the faithful transmission of chromosomes during cell division (Grewal and Elgin (2002) Curr. Opin. Genet. Dev. 12:178-187).
Further, lysine methylation catalyzed by SET domain containing proteins has been linked to cancer (Schneider, et al. (2002) Trends Biochem. Sci. 27:396-402). For example, the H3-K4 methyltransferase MLL is frequently translocated in leukemia (Ayton and Cleary (2001) Oncogene 20:5695-5707; Milne, et al. (2002) Mol. Cell 10:1107-1117; Nakamura, et al. (2002) Mol. Cell 10:1119-1128) and the H3-K27 methyltransferase EZH2 is overexpressed in a number of tumors and its expression level correlates with the invasiveness of these tumors (Bracken, et al. (2003) EMBO J. 22:5323-5335; Kleer, et al. (2003) Proc. Natl. Acad. Sci. USA 100:11606-11611; Varambally, et al. (2002) Nature 419:624-9).
Dot1 is an evolutionarily conserved protein that was originally identified in S. cerevisiae as a disrupter of telomeric silencing (Singer, et al. (1998) Genetics 150:613-632). It also functions at the pachytene checkpoint during the meiotic cell cycle (San-Segundo and Roeder (2000) Mol. Biol. Cell. 11:3601-3615). Sequence analysis of yeast Dot1 revealed that it possesses certain characteristic SAM binding motifs, similar to the ones in protein arginine methyltransferases (Dlakic (2001) Trends Biochem. Sci. 26:405-407).
It has recently been demonstrated that hDOT1L is a histone H3-K79 methyltransferase (Feng et al., (2002) Curr. Biol. 12:1052-1058), and plays an important role in MLL-AF10-mediated leukemogenesis (Okada et al., (2005) Cell 121:167-78). It was shown that mistargeting of hDOT1L to the Hoxa9 gene by MLL-AF10 results in H3-K79 methylation and Hoxa9 upregulation which contributes to leukemic transformation (Okada et al., (2005) Cell 121:167-78). It was further demonstrated that the hDOT1L and MLL-AF10 interaction involves the OM-LZ (octapeptide motif-leucine zipper) region of AF10, required for MLL-AF10-mediated leukemic transformation (DiMartino et al., (2002) Blood 99:3780-5).